Thermostats for controlling climate control systems generally fall into two broad categories: (1) mechanical thermostats and (2) electronic digital thermostats. A typical mechanical thermostat, such as for a multi-stage thermostat application, is connected to an alternating current (AC) power supply, normally a transformer. However, mechanical thermostats do not require continuous electrical power.
On the other hand, a typical digital thermostat, such as one that is multi-stage, requires continuous power for thermostat components such as a microcomputer and thermostat control circuit. These component parts require power at all times. Therefore, digital thermostats need additional connections to a power supply, such as a transformer, to satisfy these power requirements.
Thus, when replacing a mechanical multi-stage thermostat with a digital multi-stage thermostat, an additional wire is usually required to power and maintain operation of the digital thermostat. When replacing one digital thermostat with another digital thermostat, the additional power connection wire is normally present.
When installing a digital thermostat for a new application, such as when building a new house, providing continuous power for the digital thermostat by adding the necessary extra power wire is relatively easy. However, in retrofit applications, and specifically, when replacing a mechanical thermostat with a digital thermostat, providing an additional wire for electrical power can be difficult, time consuming and costly.
Therefore, what has been needed is a digital thermostat having multiple power capabilities that is adapted for both new installations and for retrofit installations (when an external power supply is not available, because all necessary connections are not present).